Currently, there are several so-called xe2x80x9clast milexe2x80x9d and xe2x80x9clast footxe2x80x9d data transmission systems which are designed to deliver high speed and/or high data capacity from the internet backbone to the end user. Several such systems use RF transmission to replace copper wire or fiber optic cables. Some of these systems are called point to point or point to multipoint systems and operate in the various licensed and unlicensed RF bands. A fundamental characteristic of such existing systems is that their RF transmissions occur in a frequency spectrum protected and regulated by a government body. These protected frequency spectrums, or bands, are licensed to certain license holders and only a few may operate in any given physical area depending upon the number of licenses available.
When operating in a licensed band the interference between transmissions is primarily self-interference and is thus controllable. Accordingly, noise (interference from another transmitter on the same frequency or on an interfering frequency) originates from a known source.
License holders are afforded protection from interference that is not self-generated which occurs within their allocated frequency band. Thus, in a protected band, if an interferer is detected, the licensed user could notify the FCC (or other regulating agency) and request that the agency investigate and rectify the problem. The regulations in the unlicensed bands differ in that systems must be designed to operate in the presence of interference and, in addition, must not generate interference. There are strict usage guidelines for the unlicensed spectrum. If an operator""s equipment in not able to tolerate interference from a system complying with the FCC guidelines, then the operator cannot appeal to the FCC to alleviate the problem.
In licensed bands, such as the cellular telephone bands; the wireless multichannel multipoint distribution service (MMDS) frequencies, which are also used for fixed wireless; or local multipoint distribution service (LMDS) used for fixed wireless; if an operator were to detect interference in the band that was not self interference, the recourse would be to contact the FCC in order to have the government identify that source of interference and have the offending provider reduce or remove that interference. These bands are licensed, have been paid for and are owned by a certain operator.
Another method for reducing interference in the licensed bands, for a cellular or fixed wireless system, is through frequency planning of the system. The only true interference of concern within these licensed bands is self-interference. So, the operator is responsible for creating a system that has acceptable levels of background interference. Traditionally, operators have mitigated interference through frequency planning, cell location and sectorization of antennas. However, use of more sophisticated techniques is generally not necessary.
There are only a limited number of licensed bands in any geographic area, thus, in order to widen the choices consumers have, it is desirable for service providers to be able to use unlicensed RF bands to provide capability to deliver high speed, high capacity data services. In addition, a service provider may not hold licenses in every geographic area that it desires to provide service. Therefore, use of the unlicensed frequency bands may allow a service provider to cover a greater area.
In 1997 the FCC created a wireless arena called Unlicensed National Information Infrastructure (U-NII). System operators are free to operate wireless equipment in three sub-bands (5.15 to 5.25 GHz, 5.25 to 5.35 GHz and 5.725 to 5.825 GHz) without acquiring a licensed frequency spectrum. Part 15 of the FCC document specifies the conditions for operating wireless equipment in the U-NII frequency band. However, operators are not protected from possible interference from other U-NII operators transmitting in the vicinity or even other systems which utilize the same frequencies.
The IEEE, a standards group, is defining a wireless LAN standard, referred to as IEEE 802.11a for operation in the U-NII band. Equipment that conforms to this standard will operate indoors at the lower and middle frequency sub-band i.e. 5.15 to 5.25 GHz and 5.25 to 5.35 GHz. The ETSI BRAN group in Europe has defined an air interface standard for high-speed wireless LAN equipment that may operate in the U-NII frequency band. Equipment that is compatible with this standard may cause interference with use of these unlicensed bands.
One major problem with the use of such unlicensed bands is that it is very difficult, if not impossible, to control RF interference from other users of the unlicensed band. These other users may be using the selected unlicensed band for uses which are essentially different from that employed to deliver communication services. For example, the 5.25 to 5.35 GHz and 5.725 to 5.825 GHz bands are available for use for outdoor data communication between two points. This is typically a wideband use. The same bands are also available for other applications including users such as government radar. When the same band is used for wideband communication, and also used by others for uses such as radar, data communications between sending and receiving antennas will experience significant interference from radar pulses, which are broadcast over a wide area in repetitive bursts.
In the current state of the art, there is no discrimination between narrow band or wideband interference. When interference is detected, it is usually based on a signal to noise ratio for any given channel, then the radio switches to a lower order modulation, from either 64QAM to 16QAM, or 16QAM to QPSK, or QPSK to BPSK. Such a lower modulation shift allows more tolerance for noise and interference, but significantly reduces the data rate. Similarly, for Orthogonal Frequency Division Multiplexing (OFDM), the modulation order of the subcarriers is optimized for any given signal to noise ratio.
The prior art of radar interference mitigation is intended for use in currently licensed RF bands. However, radar interference is not an issue of great concern in licensed bands because there is little or no such interference. Most licensed bands are free and clear of other harmful interferers originating from outside sources. Additionally, most unlicensed bands do not have strong radar interferers. However, there is other low level interference in the unlicensed RF bands. This interference is at a much lower level and has a different signature than high powered radar. Therefore, generally speaking, prior art interference mitigation systems do not detect radar interference nor do they attempt to avoid it.
An important issue in unlicensed frequency band data transmission is the large amount of interference that can be present within the system. Interference is generated within the system, as well as coming from outside sources. Interference can come from other operators in the U-NII bands, point-to-point microwave links operating in the bands, or high powered radar systems. In order to deploy a system with high through-put using these bands, it is necessary to mitigate the interference caused by these different sources. The interferences of primary concern for the present invention are point-to-point microwave links and radar pulses, due to their high power. Microwave link power can be on the order of 10 to 20 dB higher power than the output allowed for unlicensed point to multipoint systems. Radar systems can also be significantly higher powered, and thereby a destructive interference source for an unlicensed data transmission system. The two different types of interference have different impacts on the design of a RF data transmission network.
Generally speaking, these interferences are somewhat unique to unlicensed bands. It is practically impossible to keep out and control unwanted frequencies in these bands. The bands are by definition unlicensed and therefore available to anyone to use as long as the equipment complies with the FCC rules governing the unlicensed band
The creation of multiple antenna beams in the creation of specific antenna patterns, either permanently or from time to time, is well known in the art. For example, Reudink, U.S. Pat. No. 5,563,610, in part, shows such a system and is hereby incorporated by reference herein.
The present invention is directed to a system and method for point-to-multipoint RF data transmission interference mitigation using adaptive beam pattern (smart) antennas. A primary advantage of smart antennas in a fixed wireless data communication system is maximization of control over the radio frequency (RF) environment. Smart antennas can be used to increase the link signal margin by providing higher gain than conventional antennas and reduce multipath effects due to use of narrow, highly directional, antenna beams. The resultant reduction in interference, resulting in an increase in the carrier to interference (C/I) ratio, allows the use of higher levels of modulation resulting in increased through-put and/or increased range.
Fixed wireless data transmission systems operating in the unlicensed frequency bands face somewhat unique challenges. The interference present in these unlicensed bands is not necessarily a result of self interference, as common in licensed bands, but interference from foreign, outside sources. These foreign sources could include competing operators in the same band, point-to-point microwave links that are permitted to operate at higher power than point to multipoint data transmission systems, pulsed radar systems, or the like.
The interference sources of primary concern for the present invention are point-to-point microwave links and radar pulses. The present smart antenna system is able to identify the sources of interference and adapt the antenna pattern in order to mitigate the interference. The antenna pattern is adaptive in azimuth, elevation, range, and/or polarization. The adaptations differ between the uplink and downlink, and are used in combination with other interference mitigation techniques.
For example, a system, with a fixed coverage downlink channel and an adaptive uplink in which a periodic radar source interferes with communications from the subscriber units to the hub, if the location of the interference can be identified, the antenna system will place a null in the direction of the radar pulse. This way communication with other subscriber units can continue even when interference is present. Simultaneously, the hub will schedule transmissions from the subscriber that are not coincident with the radar pulse.
A continuous source of interference, such as a point-to-point radio, may result in certain portions of a cell being unavailable for coverage on a particular frequency, forcing a subscriber to communicate with an alternative hub, or changing frequencies between the subscriber and optimal hub.
The preferred method to deal with high levels of interference in the unlicensed band is through the use of smart antenna systems. Smart antennas can take different forms, with a simple smart antenna being a switched beam system. Such a system employs multiple narrow beams covering a sector. A sector may be, by way of example, 60 degrees to 120 degrees, or a full 360 degree area. Since narrow beams cover less area than a wide beam system, a reduced amount of interference is built into the network. Advantageously, the users of a fixed wireless system are, by definition, fixed and not moving. This allows scheduling of transmissions and receptions from particular users to take place on narrow beams around interference. Generally, each sector is assigned a specific frequency or set of frequencies.
More sophisticated smart antennas introduce nulls in the direction of interference. This enables communication with a user adjacent, but not in direct line with, the source of interference. For example, if a fixed point-to-point microwave link is in a particular direction from a base station, a null can be created in that direction, for that particular frequency and for that particular polarization. This allows communication with the users in other directions on that same frequency with the same polarization resulting in continued maximization of frequency use or through-put on the available band. To serve users in the direction of the interfering point-to-point source, either a different frequency or a different polarization can be used. Similar methods can be utilized for a pulse radar system, for example, where the interference is of a known periodicity, and the system reacts periodically to blank out data transmission in the direction of the interference coming from the radar.
Other irregular interference can be detected and nulls created to optimize the system to mitigate the effect of the interferences. Factors which must be taken into account for irregular interference includes the duration of the interference, the intensity and width of the interference, both in bandwidth and azimuth.
The present, sophisticated smart antenna system can steer in three dimensions. Therefore, the desired user can be located both in azimuth and elevation. The resultant communication is directly with that user and only with that user. This helps reduce self-interference throughout the network, since the data transmission system is not generating interference to different users within the system. It also helps avoid interfering with others using the unlicensed bands.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.